Mobile communication method and mobile station

ABSTRACT

The present invention relates to a mobile communication method in which a mobile station performs a handover from a handover source radio base station to a handover target radio base station. The mobile communication method includes the steps of: (A) acquiring, at the handover target radio base station, from the handover source radio base station or a switching center, a key for calculating a first key for generating a certain key used in a communication between the handover target radio base station and the mobile station; and (B) acquiring, at the handover target radio base station, from the switching center, a second key for calculating a first key for generating a certain key used in a communication between a next handover target radio base station and the mobile station.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.12/989,063 filed Oct. 21, 2010, which is a national stage application ofPCT/JP2009/061227, and claims priority to JP2008/162617.

TECHNICAL FIELD

The present invention relates to a mobile communication method forcommunicating between a mobile station and a radio base station using acertain key.

BACKGROUND ART

A conventional mobile communication system of the LTE (Long TermEvolution) scheme specified by the 3GPP is configured to communicatebetween a mobile station UE and a radio base station eNB, by using acertain key.

The certain key includes, for example, a key K_(RRC) _(—) _(Ciph) usedfor “Ciphering” in an RRC protocol, which is a C-plane protocol betweenthe mobile station UE and the radio base station eNB (Access Stratum,AS), a key K_(RRC) _(—) _(IP) used for “Integrity Protection” in the RRCprotocol, and a key K_(UP) _(—) _(Ciph) used for a U-plane protocolbetween the mobile station UE and the radio base station eNB (AccessStratum, AS) and the like. These certain keys are generated using afirst key K_(eNB).

Using the same key as any of the certain keys and the first key K_(eNB)for a long time is not preferable, because it makes the system'ssecurity vulnerable. For this reason, a procedure for updating such acertain key or a first key K_(eNB) during handover is devised by the3GPP.

Here, operations of a handover target radio base station (Target eNB)acquiring a first key K_(eNB)** used for generating a certain key in thehandover procedure of the mobile station UE are described referring toFIG. 12.

As shown in FIG. 12, first, a handover source radio base station (SourceeNB) generates an intermediate key K_(eNB)* based on a stored first keyK_(eNB), a parameter “Next Hop”, a parameter “Handover Type”representing the parameter type and a parameter “Target PCI”representing the identification information of a handover target cell.

Secondly, the handover source radio base station (Source eNB) transmitsthe generated intermediate key K_(eNB)* to the handover target radiobase station (Target eNB).

Thirdly, the handover target radio base station (Target eNB) generates,based on the the received intermediate key K_(eNB)* and “C-RNTI (CellRadio Network Temporay ID)” allocated by the handover target cell, afirst key K_(eNB)** used for generating a certain key in the handovertarget radio station (Target eNB).

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

However, as described above, in the handover procedure of theconventional mobile communication system, there is a problem that bothhandover source radio base station (Source eNB) and handover targetradio base station (Target eNB) have to use a plurality of parametersand functions to generate a first key K_(eNB)** used in the handovertarget radio station (Target eNB).

In particular, there is a problem that the handover source radio basestation (Source eNB) and the handover target radio base station (TargeteNB) have to use K_(eNB) conversion functions (Key Derivation Function,KDF) different in parameters for each of the stations, and the mobilestation UE also has be provided with the KDFs, whereby the procedure iscomplicated.

Furthermore, it is cumbersome that K_(eNB) needs to be updated accordingto PCI (Physical Cell ID) of the handover target radio base station.

Furthermore, there is a restriction in flexibly changing the allocationof C-RNTI, since K_(eNB) needs to be updated according to C-RNTI.

Accordingly, the present invention has been made in view of theabove-described problems, and an object of the present invention is toprovide a mobile communication method with which a first key used in ahandover target radio base station (Target eNB) can be generated througha simplified procedure.

Solution to Problem

A first aspect of the present invention is summarized as a mobilecommunication method in which a mobile station performs a handover froma handover source radio base station to a handover target radio basestation, the mobile communication method including the steps of: (A)acquiring, at the handover target radio base station, from the handoversource radio base station or a switching center, a key for calculating afirst key for generating a certain key used in a communication betweenthe handover target radio base station and the mobile station; and (B)acquiring, at the handover target radio base station, from the switchingcenter, a second key for calculating a first key for generating acertain key used in a communication between a next handover target radiobase station and the mobile station.

In the first aspect, the mobile communication method can further includethe step of: (C) updating, at the mobile station, upon receiving ahandover command signal from the handover source radio base station, afirst key for generating a certain key used in a communication betweenthe handover source radio base station and the mobile station, to thefirst key for generating the certain key used in the communicationbetween the handover target radio base station and the mobile station.

In the first aspect, in the step (C), the mobile station can update thefirst key for generating the certain key used in the communicationbetween the handover source radio base station and the mobile station,to the first key for generating the certain key used in thecommunication between the handover target radio base station and themobile station, based on a parameter included in the handover commandsignal.

In the first aspect, the step (C) can include the steps of: (C1)generating, at the mobile station, the first key for generating thecertain key used in the communication between the handover target radiobase station and the mobile station based on the parameter included inthe handover command signal, when the parameter is incremented; and (C2)generating, at the mobile station, the first key for generating thecertain key used in the communication between the handover target radiobase station and the mobile station based on the first key forgenerating the certain key used in the communication between thehandover source radio base station and the mobile station, when theparameter included in the handover command signal is not incremented.

In the first aspect, in the step (C1), when the parameter included inthe handover command signal is incremented, the mobile station canupdate, based on the parameter, a second key for calculating the firstkey for generating the certain key used in the communication between thehandover target radio base station and the mobile station, and cangenerate the first key for generating the certain key used in thecommunication between the handover target radio base station and themobile station based on the updated second key.

In the first aspect, the parameter can be KI.

In the first aspect, the mobile communication method can further includethe step of: (D) storing, at the mobile station, the received parameter.

A second aspect of the present invention is summarized as a radio basestation which functions as a handover target radio base station when amobile station performs a handover from a handover source radio basestation to the handover target radio base station, the radio basestation including: a first acquiring unit configured to acquire, fromthe handover source radio base station, a key for calculating a firstkey for generating a certain key used in a communication between thehandover target radio base and the mobile station; and a secondacquiring unit configured to acquire, from a switching center, a secondkey for calculating a first key for generating a certain key used in acommunication between a next handover target radio base station and themobile station.

A third aspect of the present invention is summarized as a mobilestation which performs a handover from a handover source radio basestation to a handover target radio base station, the mobile stationincluding: a key updating unit configured to update, upon receiving ahandover command signal from the handover source radio base station, afirst key for generating a certain key used in a communication betweenthe handover source radio base station and the mobile station, to afirst key for generating a certain key used in a communication betweenthe handover target radio base station and the mobile station.

In the third aspect, the key updating unit can be configured to update,based on a parameter included in the handover command signal, the firstkey for generating the certain key used in the communication between thehandover source radio base station and the mobile station, to the firstkey for generating the certain key used in the communication between thehandover target radio base and the mobile station.

In the third aspect, the key updating unit can be configured togenerate, when the parameter included in the handover command signal isincremented, the first key for generating the certain key used in thecommunication between the handover target radio base station and themobile station, based on the parameter; and the key updating unit can beconfigured to generate, when the parameter included in the handovercommand signal is not incremented, the first key for generating thecertain key used in the communication between the handover target radiobase station and the mobile station, based on the first key forgenerating the certain key used in the communication between thehandover source radio base station and the mobile station.

In the third aspect, the key updating unit can be configured to update,when a parameter included in the handover command signal is incremented,a second key for calculating the first key for generating the certainkey used in the communication between the handover target radio basestation and the mobile station, based on the parameter, and to generatethe first key for generating certain keys used in the communicationbetween the handover target radio base station and the mobile station,based on the updated second key.

In the third aspect, the parameter can be KI.

In the third aspect, the key updating unit can be configured to storethe received parameter.

Effect of the Invention

As described above, according to the present invention, it is possibleto provide a mobile communication method with which a first key used ina handover target radio base station (Target eNB) can be generatedthrough a simplified procedure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall configurational view of a mobile communicationsystem according to a first embodiment of the present invention.

FIG. 2 is a diagram showing an example of a hierarchical structure and acalculation procedure of a key used in the mobile communication systemaccording to the first embodiment of the present invention.

FIG. 3 is a sequence diagram showing an initial establishment procedurein the mobile communication system according to the first embodiment ofthe present invention.

FIG. 4 is a sequence diagram showing an X2 handover procedure in themobile communication system according to the first embodiment of thepresent invention.

FIG. 5 is a sequence diagram showing an S1 handover procedure in themobile communication system according to the first embodiment of thepresent invention.

FIG. 4 is a sequence diagram showing an Intra-eNB handover procedure inthe mobile communication system according to the first embodiment of thepresent invention.

FIG. 7 is a sequence diagram showing an S1 handover procedure in amobile communication system according to a second embodiment of thepresent invention.

FIG. 8 is a diagram showing an exemplary hierarchical structure andcalculation procedure of keys used in a mobile communication systemaccording to a third embodiment of the present invention.

FIG. 9 is a sequence diagram showing an X2 handover procedure in themobile communication system according to the third embodiment of thepresent invention.

FIG. 10 is a sequence diagram showing an S1 handover procedure in themobile communication system according to the third embodiment of thepresent invention.

FIG. 11 is a sequence diagram showing an Intra-eNB handover procedure inthe mobile communication system according to the third embodiment of thepresent invention.

FIG. 12 is a diagram showing an exemplary calculation procedure of keysused in a mobile communication system according to a conventionaltechnique.

BEST MODES FOR CARRYING OUT THE INVENTION Mobile Communication SystemAccording to First Embodiment of the Present Invention

A mobile communication system according to a first embodiment of thepresent invention is described referring to FIG. 1 to FIG. 6.

The mobile communication system according to this embodiment is a mobilecommunication system to which the LTE scheme is applied, and includes aplurality of switching centers MME#1, MME#2, . . . and a plurality ofradio base stations eNB#11, eNB#12, eNB#21, eNB#22, . . . .

For example, a mobile station UE is configured to communicate, in thecell #111 under the control of the radio base station eNB#11, with theradio base station eNB#11 using a certain key described above.

Furthermore, in the handover procedure of the mobile station UE, thehandover target radio base station (for example, the radio base stationeNB#12) is configured to acquire first keys K_(eNB)[n+1] K_(eNB)[n+2]and the like for generating certain keys used in a communication withthe mobile station UE, without using an intermediate key K_(eNB)*generated by the handover source radio base station (for example, theradio base station eNB#11).

FIG. 2 shows an example of the hierarchical structure and thecalculation procedure of a key used in the mobile communication systemaccording to this embodiment (that is, a key used to calculate thecertain key).

As shown in FIG. 2, a key K_(RRC) _(—) _(IP) used for “IntegrityProtection” in the RRC protocol, a key K_(RRC) _(—) _(Ciph) used for“Ciphering” in the RRC protocol, and a key K_(UP) _(—) _(Ciph) used for“Ciphering” in the U-plane of AS are generated using a first key K_(eNB)[n].

The first key K_(eNB)[n] is calculated by using a master key K_(ASME)from the formulas given below.

K _(emB)[0]=KDF₀(K _(ASME), NAS SN)

K _(eNB) [n+1]=KDF₁(K _(ASME) , K _(eNB) [n]), (n÷0)

Here, the master key K_(ASME) is known only to the mobile station UE andthe switching center MME, but must not be known to the radio basestation eNB.

Furthermore, the NAS SN is a sequence number (SN) of a NAS protocolwhich is the C-plane protocol between the mobile station UE and theswitching center MME (Non Access Stratum, NAS).

Hereafter, operations of the mobile communication system according tothis embodiment are described referring to FIG. 3 to FIG. 6.

First, an initial establishment procedure in the mobile communicationsystem according to this embodiment is described referring to FIG. 3.

As shown in FIG. 3, before starting the initial establishment procedure,the mobile station UE holds K_(ASME) (in step S101), the radio basestation eNB holds no keys used for generating certain keys (in stepS102), and the switching center MME holds K_(ASME) (in step S103).

In step S104, the mobile station UE transmits “RRC Connection Request(RRC connection request signal)” to the radio base station eNB, and instep S105, the radio base station eNB transmits “RRC Connection Setup(RRC connection setup signal)” to the mobile station UE.

In step S106, the mobile station UE transmits “RRC Connection SetupComplete (RRC connection setup complete signal)” to the radio basestation eNB and “NAS Service Request (NAS service request signal)”including “NAS SN (sequence number of NAS)”.

In step S107, the radio base station eNB transmits “S1 Initial UEMessage” and “NAS Service Request (NAS service request signal)”including “NAS SN” to the switching center MME.

In step S108, the switching center MME calculates K_(eNB)[0] andK_(eNB)[1] from the formulas given below.

K _(eNB)[0]=KDF₀(K _(ASME), NAS SN)

K _(eNB)[1]=KDF₁(K _(ASME) , K _(eNB)[0])

In step S109, the switching center MME transmits “S1 Initial UE ContextSetup (initial UE context setup signal)” including K_(eNB)[0],K_(eNB)[1] and “NAS SN” to the radio Base station eNB. Furthermore,“KI(=0)” may or may not be included in this message.

In step S110, the radio base station eNB transmits “RRC Security ModeCommand (RRC security mode command signal)” including “NAS SN” to themobile station UE.

In step S111, the mobile station UE calculates K_(eNB) [0] from theformula given below.

K _(emB)[0]=KDF₀(K _(ASME), NAS SN)

Furthermore, the mobile station UE calculates K_(RRC) _(—) _(IP),K_(RRC) _(—) _(Ciph) and K_(UP) _(Ciph) based on K_(eNB)[0], and usesthem in subsequent AS communications.

In this stage, the mobile station UE holds K_(eNB)[0], and “KI(=0)” (instep S114) the radio base station eNB holds K_(eNB)[0], K_(eNB)[1] and“KI(=0)” (in step S113), and the switching center MME holds K_(ASME),K_(eNB)[1] and “KI(=0)” (in step S112).

If “KI(=0)” is not included in the “S1 Initial UE Context Setup (initialUE context setup signal)” in step S109, the radio base station eNB mayinitialize “KI(=0)” automatically by receiving the above message.

Furthermore, the radio base station eNB calculates K_(RRC) _(—) _(IP),K_(RRC) _(—) _(Ciph) and K_(UP) _(—) _(Ciph) based on K_(eNB)[0], anduses them in subsequent AS communications.

In step S115, the radio base station eNB transmits “RRC ConnectionReconfiguration (RRC connection reconfiguration signal)” to the mobilestation UE.

Insteps S116 and S117, the mobile station UE respectively transmits “RRCSecurity Mode Command Complete (RRC security mode command completesignal)” and “RRC Connection Reconfiguration Complete (RRC connectionreconfiguration complete signal)” to the radio base station eNB.

In step S118, the radio base station eNB transmits “S1 Initial UEContext Setup Complete (initial UE context setup complete signal)” tothe switching center MME.

Through the above procedure, all keys necessary for protection of AScommunication (integrity protection and ciphering) are prepared at themobile station UE, the radio base station eNB and the switching centerMME.

Secondly, an X2 handover procedure (handover procedure between differentradio base stations) in the mobile communication system according tothis embodiment is described referring to FIG. 4.

As shown in FIG. 4, before starting the X2 handover procedure, themobile station UE holds K_(eNB)[n] and “KI(=n)” (in step S1001), thehandover source radio base station (Source eNB) holds K_(eNB)[n],K_(eNB)[n+1] and “KI(=n)” (in step S1002), and the switching center MMEholds K_(ASME), K_(eNB)[n+1] and “KI(=n)” (in step S1003).

In step S1004, if predetermined conditions are satisfied, the mobilestation UE transmits “RRC Measurement Report (measurement reportsignal)” to the handover source radio base station (Source eNB).

In step S1005, the handover source radio base station (Source eNB)transmits “X2 HO Preparation (handover preparation signal)” includingK_(eNB)[n+1] and “KI(=n+1)” to the handover target radio base station(Target eNB).

In step S1006, the handover target radio base station (Target eNB)stores the received K_(eNB)[n+1] and “KI(=n+1)”, and in step S1007,transmits “X2 HO Preparation Ack (handover preparation acknowledgesignal)” to the handover source radio base station (Source eNB).

Furthermore, the radio base station eNB calculates K_(RRC) _(—) _(IP),K_(RRC) _(—) _(Ciph) and K_(UP) _(—) _(Ciph) based on K_(eNB)[n+1] anduses them in subsequent AS communications.

In step S1008, the handover source radio base station (Source eNB)transmits “RRC HO Command (handover command signal)” to the mobilestation UE.

In step S1009, the mobile station UE calculates K_(eNB)[n+1] from theformula given below, and in step S1010, stores K_(eNB)[n+1] and“KI(=n+1)”.

K _(eNB) [n+1]=KDF₁(K _(ASME) , K _(eNB) [n])

Furthermore, the mobile station UE calculates K_(RRC) _(—) _(IP),K_(RRC) _(—) _(Ciph) and K_(UP) _(—) _(Ciph) based on K_(eNB)[n+1] anduses them in subsequent AS communications.

In step S1011, the mobile station UE transmits “RRC HO

Complete (handover complete signal)” to the handover target radio basestation (Target eNB).

In step S1012, the handover target radio base station (Target eNB)transmits “S1 Path Switch (path switch signal)” including “KI(=n+1)” tothe switching center MME.

In step S1013, the switching center MME calculates K_(eNB)[n+2] from theformula given below, and in step S1014, stores K_(eNB)[n+2] and“KI(=n+1)”.

K _(eNB) [n+2]=KDF₁(K _(ASME) , K _(eNB)[n+1])

In step S1015, the switching center MME transmits “S1 Patch Switch Ack(path switch acknowledge signal) ” including K_(eNB)[n+2] and “KI(=n+1)”to the handover target radio base station (Target eNB).

In step S1016, the handover target radio base station (Target eNB)stores K_(eNB)[n+1], K_(eNB)[n+2] and “KI(=n+1)”.

Through the above procedure, K_(eNB) and certain keys are updated in theX2 handover.

Thirdly, an S1 handover procedure (handover procedure between differentswitching centers) in the mobile communication system according to thisembodiment is described referring to FIG. 5.

As shown in FIG. 5, before starting the S1 handover procedure, themobile station UE holds K_(eNB)[n] and “KI(=n)” (in step S2001), thehandover source radio base station (Source eNB) holds K_(eNB)[n],K_(eNB)[n+1] and “KI(=n)” (in step S2002), and the switching center MMEholds K_(ASME), K_(eNB)[n+1] and “KI(=n)” (in step S2003).

In step S2004, if predetermined conditions are satisfied, the mobilestation UE transmits “RRC Measurement Report (measurement reportsignal)” to the handover source radio base station (Source eNB).

In step S2005, the handover source radio base station (Source eNB)transmits “S1 HO Required (handover request receipt signal)” includingK_(eNB)[n+1] and “KI(=n+1)” to the handover source switching center(source MME).

In step S2006, the handover source switching center (Source MME)transmits “Relocation Request (relocation request signal) ” includingK_(ASME), K_(eNB)[n+1] and “KI(=n+1)” to the handover target switchingcenter (Target MME).

In step S2007, the handover target switching center (Target MME)calculates K_(eNB)[n+2] from the formula given below, and in step S2008,stores K_(eNB)[n+2] and “KI(=n+1)”.

K _(eNB) [n+2]=KDF₁(K _(ASME) , K _(eNB)[n+1])

In step S2009, the handover target switching center (Target MME)transmits “S1 HO Request (handover request signal)” includingK_(eNB)[n+1], K_(eNB)[n+2] and “KI(=n+1)” to the handover target radiobase station (Target eNB).

In step S2010, the handover target radio base station (Target eNB)transmits “S1 HO Request Ack (handover request acknowledge signal)” tothe handover target switching center (Target MME).

In step S2011, the handover target switching center (Target MME)transmits “Relocation Request Ack (relocation request acknowledgesignal)” including “KI(=n+1)” to the handover source switching center(Source MME).

In step S2012, the handover source switching center (Source MME)transmits “S1 HO Required Ack (handover request receipt acknowledgesignal)” including “KI(=n+1)” to the handover source radio base station(Source eNB).

In step S2013, the handover source radio base station (Source eNB)transmits “RRC HO Command (handover command signal)” to the mobilestation UE.

In step S2014, the mobile station UE calculates K_(eNB)[n+1] from thefollowing formula, and in step S2015, stores K_(eNB)[n+1] and“KI(=n+1)”.

K _(eNB) [n+1]=KDF₁(K _(ASME) , K _(eNB) [n])

Furthermore, the mobile station UE calculates K_(RRC) _(—) _(IP),K_(RRC) _(—) _(Ciph) and K_(UP) _(—) _(Ciph) on the basis ofK_(eNB)[n+1] and uses them in subsequent AS communications.

At this stage, the handover target radio base station (Target eNB) holdsK_(eNB)[n+1], K_(eNB)[n+2] and “KI(=n+1)” (in step S2016). The radiobase station eNB calculates K_(RRC) _(—) _(IP), K_(RRC) _(—) _(Ciph) andK_(UP) _(—) _(Ciph) based on K_(eNB)[n+1], and uses them in subsequentAS communications.

In step S2017, the mobile station UE transmits “RRC HO Complete(handover complete signal)” to the handover target radio base station(Target eNB).

In step S2018, the handover target radio base station (Target eNB)transmits “S1 HO Complete (handover complete signal)” to the handovertarget switching center (Target MME).

In step S2019, the handover target switching center (Target MME)transmits “Relocation Complete (relocation complete signal)” to thehandover source switching center (Source MME), and in step S2020, thehandover source switching center (Source MME) transmits “RelocationComplete Ack (relocation complete acknowledge signal)” to the handovertarget switching center (Target MME).

Through the above procedure, K_(eNB) and certain keys are updated in theS1 handover.

Operations of the mobile station UE in the S1 handover procedure aresame as operations in the X2 handover procedure shown in FIG. 3. Basedon the same processing, the mobile station UE is capable of performingboth X2 and S1 handover procedures. That is, the mobile station UE iscapable of performing a handover regardless of whether the handover typeis “X2 handover” or “S1 handover”.

Fourthly, an Intra-eNB handover procedure (inter-radio base stationhandover procedure) in the mobile communication system according to thisembodiment is described referring to FIG. 6.

As shown in FIG. 6, before starting the Intra-eNB handover procedure,the mobile station UE holds K_(eNB)[n] and “KI(=n)” (in step S4001), theradio base station (Source eNB) holds K_(eNB)[n], K_(eNB)[n+1] and“KI(=n)” (in step S4002), and the switching center MME holds K_(ASME),K_(eNB)[n+1] and “KI(=n)” (in step S4003).

In step S4004, if predetermined conditions are satisfied, the mobilestation UE transmits “RRC Measurement Report (measurement reportsignal)” to the radio base station (Source eNB).

In step S4005, the radio base station (Source eNB) transmits “RRC HOCommand (handover command signal)” to the mobile station UE.

In step S4006, the mobile station UE calculates K_(eNB)[n+1] from theformula given below, and in step S4007, stores K_(eNB)[n+1] and“KI(=n+1)”.

K _(eNB) [n+1]=KDF₁(K _(ASME) , K _(eNB) [n])

Furthermore, the mobile station UE calculates K_(RRC) _(—) _(IP),K_(RRC) _(—) _(Ciph) and K_(UP) _(—) _(Ciph) based on K_(eNB)[n+1] anduses them in subsequent AS communications.

At this stage, the radio base station (Source eNB) holds K_(eNB)[n+1]and “KI(=n+1)” (in step S4008). The radio base station eNB calculatesK_(RRC) _(—) _(IP), K_(RRC) _(—) _(Ciph) and K_(UP) _(—) _(Ciph) basedon K_(eNB)[n+1] and uses them in subsequent AS communications.

In step S4009, the mobile station UE transmits “RRC HO Complete(handover complete signal)” to the radio base station (Source eNB).

In step S4010, the radio base station (Source eNB) transmits “S1 PathSwitch (path switch signal)” including “KI(=n+1)” to the switchingcenter MME.

In step S4011, the switching center MME calculates K_(eNB)[n+2] from theformula given below, and in step S4012, stores K_(ASME), K_(eNB)[n+2]and “KI(=n+1)”.

K _(eNB) [n+2]=KDF₁(K _(ASME) , K _(eNB) [n+1])

In step S4013, the switching center MME transmits “S1 Path Switch Ack(path switch acknowledge signal)” including K_(eNB)[n+2] and “KI(=n+1)”to the radio base station (Source eNB).

In step S4014, the radio base station (Source eNB) stores K_(eNB)[n+1],K_(eNB)[n+2] and “KI(=n+1)”. At this stage, the mobile station UE holdsK_(eNB)[n+1] and “KI(=n+1)” (in step S4015).

Through the above procedure, K_(eNB) and certain keys are updated in theIntra-NB handover.

Operations of the mobile station UE in the Intra-eNB handover procedureare same as operations in the X2 handover procedure shown in FIG. 3 andin the S1 handover procedure shown in FIG. 4. Based on the sameprocessing, the mobile station UE is capable of performing all of X2, S1and Intra-eNB handover procedures. That is, the mobile station UE iscapable of performing a handover with regardless of whether the handovertype is “X2 handover”, “S1 handover” or “Intra-eNB handover”

Advantageous Effects of Mobile Communication System According to FirstEmbodiment of the Present Invention

In the mobile communication system according to the first embodiment ofthe present invention, K_(eNB)[n+1] and the like used in the handovertarget radio base station (Target eNB) can be generated through asimplified procedure.

Furthermore, in the mobile communication system according to the firstembodiment of the present invention, there is no need to changeoperations of the mobile station UE in a handover procedure regardlessof the handover type (X2 handover, S1 handover or Intra-eNB handover).

Mobile Communication System According to Second Embodiment of thePresent Invention

Referring to FIG. 7, a mobile communication system according to a secondembodiment of the present invention is described by focusing ondifferences from the above described mobile communication systemaccording to the first embodiment of the present invention.

Specifically, the S1 handover procedure (handover procedure betweendifferent switching centers) in the mobile communication systemaccording to this embodiment is described referring to FIG. 7.

As shown in FIG. 7, operations in step S3001 to step S3006 are same asoperations in step S2001 to step S2006 shown in FIG. 5.

In step S3007, the handover target switching center (Target MME)calculates K_(eNB[n+)3] from the formulas given below, and in stepS3008, stores K_(eNB)[n+3] and “KI(=n+2)”.

K _(eNB) [n+2]=KDF₁(K _(ASME) , K _(eNB) [n+1])

K _(eNB) [n+3]=KDF₁(K _(ASME) , K _(eNB) [n+2])

In step S3009, the handover target switching center (Target MME)transmits “S1 HO Request (handover request signal)” includingK_(eNB)[n+2], K_(eNB)[n+3] and “KI(=n+2)” to the handover target radiobase station (Target eNB).

In step S3010, the handover target radio base station (Target eNB)transmits “S1 HO Request Ack (handover request acknowledge signal)” tothe handover target switching center (Target MME).

In step S3011, the handover target switching center (Target MME)transmits “Relocation Request Ack (relocation request acknowledgesignal)” including “KI(=n+2)” to the handover source switching center(Source MME).

In step S3012, the handover source switching center (Source MME)transmits “S1 HO Required Ack (handover request receipt acknowledgesignal)” including “KI(=n+2)” to the handover source radio base station(Source eNB).

In step S3013, the handover source radio base station (Source eNB)transmits “RRC HO Command (handover command signal)” to the mobilestation UE. This message may include information indicating “KI(=n+2)”.

In step S3014, the mobile station UE calculates K_(eNB)[n+2] from theformulas given below, and in step S3015, stores K_(eNB)[n+2] and“KI(=n+2)”.

K _(eNB) [n+1]=KDF₁(K _(ASME) , K _(eNB) [n])

K _(eNB) [n+2]=KDF₁(K _(ASME) , K _(eNB) [n+1])

Furthermore; the mobile station UE calculates K_(RRC) _(—) _(IP),K_(RRC) _(—) _(Ciph) and K_(UP) _(—) _(Ciph) based on K_(eNB)[n+2] anduses them in subsequent AS communications.

At this stage, the handover target radio base station (Target eNB) holdsK_(eNB)[n+2], K_(eNB)[n+3] and “KI(=n+1)” (in step S3016). The radiobase station eNB calculates K_(RRC) _(—) _(IP), K_(RRC) _(—) _(Ciph) andK_(UP) _(—) _(Ciph) based on K_(eNB)[n+2] and uses them in subsequent AScommunications.

Hereafter, operations in step S3017 to step S3020 are same as operationsin step S2017 to step S2020 shown in FIG. 5.

Through the above procedure, certain keys and K_(eNB) used in the AScommunication in the handover target radio base station (Target eNB)becomes unidentifiable to the handover source radio base station (SourceeNB), whereby system's security is improved.

Mobile Communication System According to Third Embodiment of the PresentInvention

Referring to FIG. 8 to FIG. 11, a mobile communication system accordingto a third embodiment of the present invention is described by focusingon differences from the above described mobile communication systemaccording to the first embodiment of the present invention.

FIG. 8 shows an example of the hierarchical structure and thecalculation procedure of a key used in the mobile communication systemaccording to this embodiment (that is, a key used to calculate thecertain key).

As shown in FIG. 8, a key K_(RRC) _(—) _(IP) used for “IntegrityProtection” in the RRC protocol, a key K_(RRC) _(—) _(Ciph) used for“Ciphering” in the RRC protocol, and a key K_(UP) _(—) _(Ciph) used for“Ciphering” in the U-plane of AS are generated using K_(eNB[n][m]).

K_(eNB)[n][m] is calculated by using K_(eNB)[n] from the formulas givenbelow.

K_(eNB)[n][0]=K_(eNB)[n]

K _(eNB) [n][m+1]=KDF₂(K _(eNB)[n][m])(m≧0)

Furthermore, K_(eNB)[n] is calculated from the formulas given belowusing K_(ASME).

K _(eNB)[0]=KDF₀(K _(ASME), NAS SN)

K _(eNB) [n+1]=KDF₁(K _(ASME) , K _(eNB) [n]), (n≧0)

Hereafter, operations of the mobile communication system according tothis embodiment are described referring to FIG. 9 to FIG. 11.

Firstly, an X2 handover procedure (handover procedure between differentradio base stations) in the mobile communication system according tothis embodiment is described referring to FIG. 9.

As shown in FIG. 9, before starting the X2 handover procedure, themobile station UE holds K_(eNB)[n], K_(eNB)[n][m], “KI(=n)” and “RC(=m)”(in step S6001), the handover source radio base station (Source eNB)holds K_(eNB)[n], K_(eNB)[n+1], K_(eNB)[n][m], “KI(=n)” and “RC(=m)” (instep S6002), and the switching center MME holds K_(ASME), K_(eNB)[n+1]and “KI(=n)” (in step S6003).

In step S6004, if predetermined conditions are satisfied, the mobilestation UE transmits “RRC Measurement Report (measurement reportsignal)” to the handover source radio base station (Source eNB).

In step S6005, the handover source radio base station (Source eNB)transmits “X2 HO Preparation (handover preparation signal)” includingK_(eNB)[n+1] and “KI(=n+1)” to the handover target radio base station(Target eNB).

In steps 56006 and 56007, the handover target radio base station (TargeteNB) stores K_(eNB)[n+1], K_(eNB)[n+1][0], “KI(=n+1)” and “RC(=0)”.Here, it is assumed that K_(eNB)[n+1][0]=K_(eNB)[n+1].

In step S6008, the handover target radio base station (Target eNB)transmits “X2 HO preparation Ack (handover preparation acknowledgesignal)” to the handover source radio base station (Source eNB).

In step S6009, the handover source radio base station (Source eNB)transmits “RRC HO Command (handover command signal)” including“KI(=n+1)” and “RC(=0)” to the mobile station UE.

In step S6010, the mobile station UE calculates K_(eNB)[n+1] andK_(eNB)[n+1][0] from the formulas given below, and in step S6011, storesK_(eNB)[n+1], K_(eNB)[n+1][0], “KI(=n+1)” and “RC(=0)”.

K _(eNB) [n+1]=KDF₁(K _(ASME) , K _(eNB) [n])

K _(eNB) [n+1][0]=K _(eNB) [n+1]

Furthermore, the mobile station UE calculates K_(RR) _(—) _(CIP), K_(RR)_(—) _(Ciph) and K_(IP) _(—) _(Ciph) based on K_(eNB)[n+1][0] and usesthem in subsequent AS communications.

Hereafter, operations in step S6012 to step S6017 are same as operationsin step S1011 to step S1016 shown in FIG. 4.

Secondly, an S1 handover procedure (handover procedure between differentswitching centers) in the mobile communication system according to thisembodiment is described referring to FIG. 10.

As shown in FIG. 10, before starting the S1 handover procedure, themobile station UE holds K_(eNB)[n], K_(eNB)[n][m], “KI(=n)” and “RC(=m)”(in step S7001), the handover source radio base station (Source eNB)holds K_(eNB)[n], K_(eNB)[n+1], K_(eNB)[n][m], “KI(=n)” and “RC(=m)” (instep S7002), and the switching center MME holds K_(ASME), K_(eNB)[n+1]and “KI(=n)” (in step S7003).

Hereafter, operations in step S7004 to step S7012 are same as operationsin step S2004 to step S2012 shown in FIG. 4.

In step S7013, the handover source radio base station (Source eNB)transmits “RRC HO Command (handover command signal)” including“KI(=n+1)” and “RC(=0)” to the mobile station UE.

Here, in step S7014, the handover target radio base station (Target eNB)calculates K_(eNB)[n+1][0] from the formula given below and stores it.

K _(eNB[n+1][0]) =K _(eNB[n+1])

At this stage, it is assumed that the handover target radio base station(Target eNB) stores K_(eNB)[n+1], K_(eNB)[n+2], K_(eNB)[n+1][0],“KI(=n+1)”, and “RC(=0)” (in step S7015). The radio base station eNBcalculates K_(RRC) _(—) _(IP), K_(RRC) _(—) _(Ciph) and K_(UP) _(—)_(Ciph) based on K_(eNB)[n+1][0] and uses them in subsequent AScommunications.

In step S7016, the mobile station UE calculates K_(eNB)[n+1] andK_(eNB)[n+1][0] from the formulas given below, and in step S7017, storesK_(eNB)[n+1], K_(eNB)[n+1][0], “KI(=n+1)” and “RC(=0)”

K _(eNB) [n+1]=KDF₁(K _(ASME) , K _(eNB) [n])

K _(eNB) [n+1[0]=K _(eNB) [n+1]

Furthermore, the mobile station US calculates K_(RRC) _(—) _(IP) K_(RRC)_(—) _(Ciph) and KUP _(—) _(Ciph) based on K_(eNB)[n+1][0] and uses themto subsequent AS communications.

Hereafter, operations in step S7018 to step S7021 are same as operationsin step S2017 to step S2020 shown in FIG. 5.

Thirdly, an Intra-eNB handover procedure (inter-radio base stationhandover procedure) in the mobile communication system according to thisembodiment is described referring to FIG. 11.

As shown in FIG. 11, before starting the Intra-eNB handover procedure,the mobile station UE holds K_(eNB)[n], K_(eNB)[n][m], “KI(=n)” and“RC(=m)” (in step S5001), the radio base station (Source eNB) holdsK_(eNB)[n], K_(eNB)[n+1], K_(eNB)[n][m], “KI(=n)” and “RC(=m)” (in stepS5002), and the switching center MME holds K_(ASME), K_(eNB)[n+1] and“KI(=n)” (in step S5003).

In step S5004, if predetermined conditions are satisfied, the mobilestation UE transmits “RRC Measurement Report (measurement reportsignal)” to the radio base station (Source eNB).

In step S5005, the radio base station (Source eNB) transmits “RRC HOCommand (handover command signal)” including “KI(=n)” and “RC(=m+1)” tothe mobile station UE.

In step S5006, the radio base station (Source eNB) calculatesK_(eNB)[n][m+1] from the formula given below, and in step S5007, storesK_(eNB)[n]K_(eNB)[n+1], K_(eNB)[n][m+1], “KI(=n+1)” and “RC(=m+1)”.

K _(eNB) [n][m+1]=KDF₂(K _(eNB) [n][m])

Furthermore, the radio base station eNB calculates K_(RRC) _(—) _(IP),K_(RRC) _(—Ciph) and K_(UP) _(—) _(Ciph) based on K_(eNB)[n][m+1] anduses them to subsequent AS communications.

At the same time, in step S5008, the mobile station UE calculatesK_(eNB)[n][m+1] from the formula given below, and in step S5009, storesK_(eNB)[n], K_(eNB)[n][m+1], “KI(=n+1)” and “RC(=m+1)”.

K _(eNB) [m+1]=KDF₂(K _(eNB) [n][m])

Furthermore, the mobile station UE calculates K_(RRC) _(—) _(IP),K_(RRC) _(—) _(Ciph) and K_(UP) _(—) _(Ciph) based on K_(eNB)[n][m+1]and uses them in subsequent AS communications.

In step S5010, if predetermined conditions are satisfied, the mobilestation UE transmits “RRC HO Complete (handover complete signal)” to theradio base station (Source eNB).

According to this embodiment, “Path Switch” in the Intra-eNB handoverprocedure can be omitted.

As shown in FIG. 9 to FIG. 11, by introducing K_(eNB) updating in theradio base station using the parameter “RC”, K_(eNB) can be updatedwhile omitting an inquiry to the switching center MME.

Meanwhile, in the procedures shown in FIG. 9 to FIG. 11, the parameter“RC” maybe omitted from “RRC HO Command (handover command signal)”.

When the parameter “RC” is omitted from “RRC HO Command (handovercommand signal)”, necessity of incrementing “RC” can be determined bydetermining whether the parameter “KI” has been incremented or not.

If the “KI” has been incremented, “RC” may be reset to “0”, whereas ifthe “KI” has not been incremented, “RC” may be incremented.

Alternatively, if the parameter “RC” is omitted from “RRC

HO Command (handover command signal)”, the mobile station UE may, on atrial basis, maintain the present value of “RC”, increment “RC” or reset“RC” to “0” and then check “Integrity” with respect to a messagereceived for each of the cases to autonomously determine which one ofthe cases is correct.

Note that operation of the above described switching center MME, theradio base station eNB and the mobile station UE may be implemented bymeans of hardware, a software module executed by a processor, or acombination of both.

The software module may be provided in any type of storage medium suchas an RAM (Random Access Memory), a flash memory, a ROM (Read OnlyMemory), an EPROM (Erasable Programmable ROM), an EEPROM (ElectronicallyErasable and Programmable ROM), a register, a hard disk, a removabledisk, or a CD-ROM.

The storage medium is connected to the processor so that the processorcan read and write information from and to the storage medium. Also, thestorage medium may be integrated into the processor. Also, the storagemedium and the processor may be provided in an ASIC. The ASIC may beprovided in the switching center MME, the radio base station eNB and themobile station UE. Also, the storage medium and the processor may beprovided in the switching center MME, the radio base station eNB and themobile station UE as a discrete component.

Hereinabove, the present invention has been described in detail usingthe above embodiment; however, it is apparent to those skilled in theart that the present invention is not limited to the embodimentdescribed herein. Modifications and variations of the present inventioncan be made without departing from the spirit and scope of the presentinvention defined by the description of the scope of claims. Thus, whatis described herein is for illustrative purpose, and has no intentionwhatsoever to limit the present invention.

1.-14. (canceled)
 15. A mobile communication method in which a mobilestation performs a handover from a handover source radio base station toa handover target radio base station by an interface via a switchingcenter, the mobile communication method comprising the steps of: (A)acquiring, at the handover target radio base station, from the switchingcenter, a first key for generating a certain key used in a communicationbetween the handover target radio base station and the mobile station;and (B) generating, at the mobile station, a first key for generating acertain key used in a communication between the handover target radiobase station and the mobile station, based on a parameter obtained byincrementing a parameter which is included in a handover command signaland which is used upon creating a first key used in a communicationbetween the handover source radio base station and the mobile station,when receiving the handover command signal from the handover sourceradio base station.
 16. A mobile station which can perform a handoverfrom a handover source radio base station to a handover target radiobase station by an interface via a switching center, wherein the mobilestation is configured to generate a first key for generating a certainkey used in a communication between the handover target radio basestation and the mobile station, based on a parameter obtained byincrementing a parameter which is included in a handover command signaland which is used upon creating a first key used in a communicationbetween the handover source radio base station and the mobile station,when receiving the handover command signal from the handover sourceradio base station.